Top class piece? Back bone? (5)
I believe the answer is:
tibia
'bone?' is the definition.
(I know that tibia is a bone)
'top class piece? back' is the wordplay.
'top class' becomes 'AI' (resembles Ab, old-fashioned term meaning excellent).
'piece?' becomes 'bit' (synonyms).
'back' says the letters should be written in reverse.
'ai'+'bit'='aibit'
'aibit' backwards is 'TIBIA'.
(Other definitions for tibia that I've seen before include "Part of leg of" , "The large shinbone" , "Bone of the lower leg" , "ancient pipe" , "part of skeleton" .)